The productivity of ecological systems
is highly sensitive to climate change and projections of change in productivity
range from increases to decreases (medium confidence). Increasing
CO2 concentrations would increase net primary productivity (CO2
fertilization) and net ecosystem productivity in most vegetation systems,
causing carbon to accumulate in vegetation and soils over time. Climate
change may either augment or reduce the direct effects of CO2
on productivity, depending on the type of vegetation, the region, and the
scenario of climate change.

The terrestrial ecosystems at present
are a carbon sink which may diminish with increased warming by the end of
the 21st century (see Table 3-2) (medium
confidence). The terrestrial ecosystems at present are a sink
for carbon. This is partly a result of delays between enhanced plant growth
and plant death and decay. Current enhanced plant growth is partly due to
fertilization effects of elevated CO2 on plant photosynthesis
(either directly via increased carbon assimilation, or indirectly through
higher water-use efficiency), nitrogen deposition (especially in the Northern
Hemisphere), climate change, and land-use practices over past decades. The
uptake will decline as forests reach maturity, fertilization effects saturate
and decomposition catches up with growth, and possibly through changes in
disturbance regimes (e.g., fire and insect outbreaks) mediated through climate
change. Some global models project that the net uptake of carbon by terrestrial
ecosystems will increase during the first half of the 21st century but may
diminish and even become a source with increased warming towards the end
of the 21st century.

Lengthening of growing season in mid- and high latitudes;
shifts in ranges of plant and animal species (high confidenced).e,f
Increase in net primary productivity of many mid- and high-latitude
forests (medium confidenced). Increase in frequency
of ecosystem disturbance by fire and insect pests (high confidenced).

Extinction of some endangered species; many others
pushed closer to extinction (high confidenced).
Increase in net primary productivity may or may not continue. Increase
in frequency of ecosystem disturbance by fire and insect pests (high
confidenced).

Loss of unique habitats and their endemic species
(e.g., vegetation of Cape region of South Africa and some cloud forests)
(medium confidenced). Increase in frequency of
ecosystem disturbance by fire and insect pests (high confidenced).

* Refer to footnotes
a-d accompanying Table 3-1.e. Aggregate market effects represent the net effects
of estimated economic gains and losses summed across market sectors
such as agriculture, commercial forestry, energy, water, and construction.
The estimates generally exclude the effects of changes in climate
variability and extremes, do not account for the effects of different
rates of change, and only partially account for impacts on goods and
services that are not traded in markets. These omissions are likely
to result in underestimates of economic losses and overestimates of
economic gains. Estimates of aggregate impacts are controversial because
they treat gains for some as canceling out losses for others and because
the weights that are used to aggregate across individuals are necessarily
subjective.f. These effects have already been observed and are
expected to continue [TAR
WGII Sections 5.2.1, 5.4.3,
16.1.3, & 19.2].